Rotary potentiometers



Dec. 25, .1962 w. J. MAlRs 3,070,768

ROTARY POTENTIOMETERS Filed March L8, 1960 2 Sheets-Shoe?I 1 INVENTOR.

WILLIAM J. MAIRS BY 71%, MM

ATTORNEYS Dec. 25, 1962 W. J. MAlRs v 3,070,768

ROTARY POTENTIOMETERS WILLIAM J. MAIRS ATTORNEYS BY @MM United States Patent O 3,070,768 ROTARY POTENTIOMETERS Wiiliam J. Mairs, Waltham, Mass., assignor to Acton Laboratories, Inc., Acton, Mass., a corporation of Massachusetts Filed Mar. 18, 1960, Ser. No. 16,048 7 Claims. (Cl. 338-154) This invention relates to variable resistors and more particularly to rotary potentiometers comprising a resistance wire wound about `a flat card that is bent into cylindrical shape.

' Heretofore I have invented ia novel form of variable resistor which is disclosed and claimed in U.S. Patent No. 2,862,089, issued to me on November 25, 1958. That novel form of variable resistor utilizes a conductive resilient l-ayer overlying but spaced from ia resistance element mounted on a dat base, a nonconductive resilient layer overlying the conductive layer in close proximity thereto, and a movable pressure foot in engagement with the nonconductive layer, the pressure foot when moved actin-g through the nonconductive layer to press successive portions of the conductive layer down in-to engagement with successive por-tions of the resistance element. ln the aforesaid construction, the conductive layer functions like the movable contact of a conventional potentiometer. While the specific constructions shown in my previous patent have many advantages, including avoidance of abrasion to the resistance element, they do not lend themselves to use of cylindrical wire-wound resist- `ance cards which are relatively cheap and easy to manufacture and which have established over the years a reputation for precision, reliability, and ease of replaceabili-ty.

Accordingly, the primary object of the present invention is to provide `an improvement on the invention claimed in my aforesaid Patent No. 2,862,089, the improvement making use of the essential principles described in that patent but involving new structural features which make possible and include the use of cylindrical wire-wound resistance cards.

In the attainment of this object I make use of two concentric cylindrical elements which define lan annular space in which is positioned and secured a cylindrical wirewo-und resistance card, a contact laminate secured to corresponding ends of the two cylindrical elements and spanning 4or bridging the annular space between the two cylindrical elements, and a rotatable pressure foot movable yalong the contact laminate and causing the latter to edge wipe, i.e., contact the edge of, the resistance element. Other objects and many of the `attendant advantages of my inventive improvement will become readily apparent las reference is had .to the following specification together with the accompanying drawings, wherein:

PEG. 1 is a cross-sectional view of a rotary potentiometer embodying one form of my new invention, the section through lthe two concentric cylindrical elements being 4taken at a position indicated by line 1-1 in FIG. 3;

FIG. 2 relates to the embodiment of FIG. 1 and is an exploded perspective view of the inner and outer cylindrical supports, the resistance element, the contact laminate, and the two standoff rings for the contact laminate;

FIG. 3 is :a perspective view showing the components of FIG. 2 in assembled relation, with a section of the contact laminate 'broken away to reveal the edge of the resistance element.

FIG. 4 is `an exploded perspective view of a second and preferred form of the invention; and

FIG. 5 is ya cross-sectional view of a rotary potentiometer embodying said second form of the invention.

Turning first to FIG. l, the first for-rn of my present ICC invention comprises a rst rigid cylindrical support element 2 having smooth exterior and in-terior surfaces 4 and 6 respectively, flat top and bottom end surfaces 8 and 10 respectively, a beveled surface 12 at the junction of top end surface 8 and exterior surface 4, and a peripheral flange 14 of rectangular cross-section at its bottom end. Interior surface 6 is provided with an axially exten-ding groove 16 which joins with a radially extending groove 18 formed in the bottom end surface 10.

Sized to surround cylindrical support element 2 is a resistance element 20 comprising a ilat card 22 which is bent into cylindrical shape as shown for installation between support element 2 and a second support element 34 described hereinafter. Card 22 is formed of a suitable material such yas sheet plastic or plastic-impregnated papel'. Wound on card 22 is =a resistance wire 24. The drawings exaggerate the spacing of the turns of wire 24; in practice, the turns are wound closely together with a predetermined number of turns per unit length. The two ends of wire 24 are spaced from each other and are provided with soldered or welded wire leads 26 and 28. Resistance element 20 -ts snugly on cylindrical support 2, ,and its bottom edge 30 seats on and is supported by flange 14. A suitable cement (not shown), such as epoxy resin cement, is applied to exterior surface 4 of cylindrical support 2 before resistance element 20 is slipped in place. The cement holds the resistance element securely in place.

This first form of my invention also comprises a second cylindrical element 34. This, like the first cylindrical element '2, is formed of rigid insulating material or of rigid conductive material protected with an overcoat of insulating material. The inner surface 36 of this second cylindrical element is sized to t snugly about resistance element 20 when Ithe latter is in cylindrical form as shown, and is undercut as shown at 38 to accommodate flange 14 of the inner element 2. The top and bottom end surfaces 40 and 42 respectively are flat, but a beveled surface 44 is formed at the juncture of inner surface 36 and top end surface 40. Outer surface 46 is-undercut at the bottom end of cylindrical element 34 so `as to form -a shoulder 48 and a second outer cylindrical surface 50. Secured to this surface are three circumferentially spaced soldering terminals 52, 54, and 56. A series of uniformly spaced slots 5S are formed in outer surface 46 adjacent shoulder 48, and a radial groove 60 is cut in undersurface 42. In addition, two holes 62 and 64 are cut completely through the wall of cylindrical element 34. These holes are cut so `as to intersect two slots 58a and 58b located close to terminals 52, 54, and 56.

A suitable cement (not shown) applied to the inner surfaces 36 .and 38 secures outer element 34 to resistance element 20 and also to inner cylindrical support 2. The cement must be compatible with the resistance element and the inner and outer supports. An epoxy resin cement has been used satisfactorily. The outer cylindrical element 34 is oriented so that its groove 60 is in line with groove 18 of the inner element 2. The wire leads 26 and 28 project through holes 62 and 64 respectively.

The axial lengths of inner and outer elements 2 and 34 are the same; the axial lengths of flange 14 and surface 38 are also the same. Hence when the two elements are assembled one upon the other, their top surfaces 8 and 40 lie in the same plane. The two beveled surfaces 12 and 44 form a substantially V-shaped valley or groove, and Ythe height, i.e., axial length, of resistance 20 is suiciently great to make it protrude into the valley but preferably not great enough to project above surfaces 8 and 40. FIGS. 1 and 3 illustrate how the top edge 66 of the resistance element extends into the valley or groove formed by beveled surfaces 12 and 44. Y

amamos Thereafter, standoff rings or gaskets 70 and 72 of suitable insulating material are secured by a suitable cement to the top end surfaces 8 and 40 respectively of the two cyindrical elements. Preferably, these rings have the same width as the surfaces to which they are secured so as to facilitate lining them up. The inner gasket 70 is provided with a slot 74 which is lined up with groove 16.

On top of gaskets 70 and 72 is placed a circular contact laminate 76 having a center hole 78 coinciding substantially with the center hole or bore in inner cylindrical support 2. Preferably, the contact laminate comprises a sheet 80 of flexible conductive material, preferably a thin metal foil such as gold, silver, or aluminum, and a sheet of flexible, resilient, relatively strong insulating material 82 such as glass fabric or nylon cloth or polyethylene sheeting, with the metal foil adhesively secured to the overlying flexible insulating material. Alternatively, the laminate may comprise a sheet of flexible, resilient fabric having a coating of conductive metal paint, eg., silver paint, on its underside. The contact laminate is secured to the assembly by means of a suitable adhesive (not shown) which is applied to the top surfaces of the two gaskets 70 and 72. Preferably, but not necessarily, the metal foil has a short tab 84 on its inner edge. When attaching the contact laminate tothe gaskets, it is oriented so that tab 84 is in registration with groove 74. In addition, the contact laminate is stretched out smooth and relatively taut so that it does not sag down into contact with the top edge 66 of the resistance element.

Thereafter the resistance ends 26 and 28 are bent down into the slots 58 located below holes 62 and 64 and are secured to solder terminals 52 and 56 respectively. Then a piece of conductor wire 86 is soldered to terminal 54 and bent downward and inward so as to seat in and run along slots 60 and 18 and bent upward so as to seat in groove 16 and slot 74. The free end of this wire 86 is soldered to tab 84 of the gold foil. To terminals 52, 54, and 56 are then soldered insulated wires 90, 92, and 94 respectively.

The foregoing assembly is now ready for installation in a, conventional potentiometer housing, such as housing 96 which has an integral bottom end wall 98 and a removable top end wall or cover 100. The outer surface of the outer support element 34 bears snugly against the inner surface of housing 96 and thereby assures concentricity of the assembly. Wires 90, 92, and 94 are put through a hole 102 formed in its side wall and secured to terminals 106, 104, and 108 respectively carried by a terminal board 110 secured to the outside surface of the housing. Thereafter a bushing 116 is inserted in the housing. Its leading end is threaded and screwed into a. tapped hole 118 formed at the center of end wall 98. Bushing 116 is screwed up tight so that its flange 120 will act on the contact laminate to hold the entire resistance assembly tight against end wall 98. Potting may be added at 122 to help keep the outer edge of the contact laminate tight against the outer standoff gasket and also to provide a secure hermetic seal. lThereafter a shaft 124 having a rotor arm 126 secured to one end is inserted in bushing 116. Arm 126 has a pressure foot 128 adjacent its free end. A spacer bearing 130 is slipped onto the shaft before it is inserted in the bushing. As its name suggests, spacer bearing 130 spaces rotor arm 126 from the adjacent end surface of the bushing and also provides a limited bearing surface for the rotor arm. A retaining ring 132, preferably of the C-shaped type, is slipped over the shaft and coacts with the bushing to keep -rotor arm 126 in slipping engagement with spacer bearing130.

Pressure foot 128 and the top exposed edge 66 of the resistance element are displaced equal distances from the center axis of shaft 124. Hence, pressure foot 128 engages the contact laminate directly above the top edge 66 of the resistance element. The length of pressure foot 128 is just suflicient to force lthe contact laminate into Cir engagement with the edge 66. In addition, the end of the pressure foo-t is relatively small and round so as to press only a limited amount of the laminate into engagement with the resistance element at any one time and also so that the pressure foot will slide easily along the laminate without scratching or gouging it.

The embodiment of FIG. l may be used as a poten tiometer o-r as a simple variable resistor. In the formerv case, an input voltage is applied across terminals and 108 and a readout voltage is obtained at terminal 104. When used as a simple variable resistor, one of the terminals 106 and 108 is not used.

The advantages of the foregoing construction are nurnerous. First and foremost is the fact that it Aprovides a very practical and new way of applying the principles disclosed in my previous Patent No. 2,862,089 to conventional wirewound cards which are far easier and cheaper to construct and use than most other forms of `resistance elements useable in potentiometers; yet they are fully as precise, have a desirable low temperature coeicient, and extremely long rotational life with low noise and little, if any, change in total resistance. Another advantage is that the wire-wound card is supported on its inner and outer faces and also its bottom edge by the two cylindrical elements 2 and 34, thereby reducing the likelihood of damage due to shock. In conventional rotary potentiometers, the inner face of wire-wound cards is left exposed for direct contact by the wiper. In addition, since the contact laminate contacts only the relatively narrow edge of the resistance card, the amount of resistance wire actually engaged by the contact laminate at any instant is quite small and, for a given pressure foot size, is even smaller than is the case with a flat resistance element having a relatively broad face. Still another advantage is obtained by slots 58. Holes such as holes 62 and 64 may be drilled at a plurality of points about cylindrical element 34, with slots 58 providing accessways for additional wire leads running from these holes to below shoulder 48. Below shoulder 48 these wire leads can be extended around surface 50 to the soldering terminals 52, 54, and 56, or to other extra soldering terminals, so that all connections to the terminal board can be made through a single hole in the housing wall. Be cause of the ease with which extra leads and extra soldering terminals can be secured to cylindrical support 34, it is a simple matter to provide extra taps to the resistance element.

It is to be noted that the foregoing advantages are yin addition to the advantages inherent in employing the principles outlined and claimed in my prior patent.

It is to be noted that the gaskets or standoff rings 70 and 74 are not absolutely necessary and may be omitted if desired. However, if the gaskets are omitted, then the top edge 66 of the resistance element must be well below the top surfaces 8 and 40 of the two concentric supports so as to prevent the contact laminate from engaging it except where forced down by the pressure foot. The gaskets simply make it easier to obtain the desired spacing between the contact laminate and the resistance element.

The second and preferred form of the invention is Shown in FIG. 5. This second form comprises an inner cylindrical support 150, a resistance element 152, an insulating ring 154, an outer cylindrical support 156,. inner and outer standoff rings 158 and 160, and a contact laminate 162 which is of the same construction as contact laminate 76 illustrated in FIG. 1. Inner support is a non-conductive member and comprises a cylindrical outer Isurface 164 whose axial length is substantially the same as the axial length of resistance element 152, a peripheral end flange 166 which is stepped to provide an annular shoulder 168 whose outside diameter is slightly less than the inside diameter of outer support 156, and an inner surface 170 which is stepped to provide a concentric reduced diameter surface 172 and an 'annular shoulder 174. Inner surface 170 is grooved axially as at 176 to expose a bore 178 which extends from the level of shoulder 174 to the bottom end of the support. Actually, resistance element 152 is in flat form until time for installation. Then it is bent into a cylinder with its ends in close spaced or butted relation as in FIG. 4. When bent into cylindrical form, its inner diameter is materially larger than the outer surface 164 of inner support 150. The insulating ring 154 is made of a suitable plastic material such as a polyethylene terephthalate or a melamine resin, or paper coated with a plastic material having insulating property, and is provided with a pair of holes 180 and 182 to accommodate leads 184 and 186 which are soldered to the ends of the resistance element during the assembly operation.

The outer support 156 is a nonconductive member and is substantially identical in shape to its counterpart 34 in FIG. 1 except for certain differences noted hereinafter. One difference is that its inside surface 188 is not stepped at its bottom end. The second difference is that its top inside corner is no-t beveled; instead, a rectangular groove 190 is provided. The diameter of inside surface 188 is just large enough to snugly receive insulating ring 154. The width of groove 190 may be varied substantially without affecting operation of the invention. Two relatively large holes 192 and 194 are provided to accommodate resistance leads 184 and 186, and a plurality of small holes 196 are provided to accommodate small soldering lugs or rivets 198 (FIG. 5).

Preferably, the Widths of standoff rings 158 and 160 are suicient to cover the top end surfaces of supports 150 and 156. However, inner ring 158 is notched as at 200 to match the cross-section of groove 176. These two standoff rings are also formed of insulating material.

The contact laminate 162 has a tongue 202 on its inner surface. This tongue is long enough to extend down into groove 176 when the various elements are assembled in the manner now to be described.

The lirst step in the assembling operation is to fit thc insulating ring 154 into the outer support 156 and position it so that its upper edge will be below the top or contact edge of the resistance element. In practice, this means that the top edge of the insulating ring will be slightly below the shoulder formed by groove 190, as in FIG. 5. The insulating ring is bonded to the inner surface of outer support 156 by means of a suitable cement. Then the resistance element 152 is inserted within insulating ring 154 with the ends of its resistance wire lined up with holes 180 and 182. The resistance element is positioned with its upper edge located at or slightly above the level of the shoulder formed by the bottom of groove 190. The resistance element is cemented to the insulating ring by a suitable quick-drying cement compatible with the insulation ring.

Thereafter the inner support 150 is inserted within the resistance element. Although FIG. 5 shows the bottom edge of the resistance element engaging shoulder 168 of the inner support, it is not necessary for this to occur. In practice, it is preferred that the axial length of the resistance element be limited just enough to prevent it from being engaged by shoulder 168. This helps assure against damage to the resistance element during assembly. The inner support is cemented to the outer support so as to make a solid unit. In this connection, it is to be noted that shoulder 168 ts snugly within the outer support so as to eliminate any side play while the two supports are being cemented together.

The next step is to apply the standoff rings by means of a suitable cement. Thereafter the contact laminate is laid over and cemented to the standoff rings. The contact laminate is pressed smooth so as to be free of any sag. At this point, there exists a unitary package or assembly which is ready for mounting in a suitable housing, as in FIG. 5.

Thereafter leads 184 and 186 are inserted through holes 192, 180 and 194, 182 respectively and soldered or welded to the ends of the resistance wire of resistance element 152. The opposite ends of leads 184 and 186 are soldered to separate terminals 198. Another lead 206 is inserted through bore 178 and secured to tongue 202 which has been bent down into groove 176 (as in FIG. 5). The other end of this lead is soldered to another one of the soldering terminals 198. The bottom ends of the inner and outer supports are slotted to provide a groove or passageway 208 whereby lead 206 can be brought out to one of the soldering terminals 198. External leads 210 are soldered to terminals 198 to connect the assembly into an electrical circuit.

The assembly can be accommodated by a housing 212 similar to the one shown in FIG. 1. However, it need not be clamped in place by a bushing as in FIG. l. Althrough a bushing 214 is shown in FIG. 5, its function is solely to support an operating shaft 216. The resistance assembly is secured in place by other means, namely, screws 218 which project through suitable holes in an end wall 220 of the housing and are screwed into suitable tapped holes provided in the bottom end surface of inner support 150.

Shaft 216 carries a radial cam 224 having a contact element 226 in engagement with contact laminate 162 directly in line with the resistance element 152. A limited area of the contact laminate is pressed into contact with the adjacent top edge of the resistance element and, as the operating shaft is rotated, different successively occurring areas of the contact laminate engage the resistance element at different points. If desired, stop means may be provided so as to limit rotation of shaft 216 to slightly less than one complete revolution. Also, if desired, potting compound may be applied over the outer edge of the contact laminate and the adjacent inner surface of the housing to completely ll in any space therebetween.

It is to be noted that the peripheral flange 166 on the inner support 150 is sized to t snugly within housing 212 whereby to assure concentricity of the assembly, including operating shaft 216.

It has been found that the best contact laminate is the one which uses as its insulating cover layer glass fiber cloth impregnated with tetrauoroethylene plastic. The material is strong, resilient, and long lasting. Tests have indicated that this material has a useful life in excess of 40 million cycles.

The depth of groove need not be great. In practice. the depth may be as little as .020 inch without any significant change in results. Similarly, the annular space between the resistance element and the outer surface of inner support may be varied considerably. A space as wide or wider than the lateral width of the groove may be used to permit proper flexure of the contact laminate.

It is to be understood that the term wire wound card as used herein denotes a resistance element comprising a thin-walled cylindrical support and a resistance wire wound on the support, with each turn of the wire extending generally longitudinally and running around the opposite end edge of the cylindrical support. The support may be a seamless cylinder or a at card bent into the shape of a cylinder after the resistance wire has been wound thereon.

Obviously, many modifications and variations of my present invention are possible in the light of the foregoing teachings. It is to be understood, therefore, that the invention is not limited in its application to the details of construction and arrangement of parts specifically described or illustrated, and that within the scope of the appended claims, it may be practiced otherwise than as specifically described or illustrated.

'I claim:

1. A variable resistor comprising a first cylindrical element, a second hollow cylindrical element surrounding and secured to said first cylindrical element, said first element having a first fiat end surface, said second element having a second flat end surface which is coplanar with said first end surface, a resistance element comprising a supporting cylinder and an elongated resistance wire wound on said cylinder with the turns of said wire extending generally longitudinally of the cylinder, said resistance element disposed in an annular space between said first and second cylindrical elements, said resistance element having an end edge located almost flush with said end surfaces, a resilient contact laminate in overlying spaced parallel relation to said end edge, said laminate having a first inner portion supported by said first end surface and a second outer portion supported by said second end surface, said laminate comprising a resilient conductive contact layer and a resilient backing for said conductive layer, movable means for selectively engaging different portions of said backing and pressing said backing toward said end edge just suicient to press said conductive layer into contact with said end edge, means for connecting said resistance wire to a voltage source, and a terminal means for connecting said conductive layer to an electrical circuit.

2. A variable resistor as defined by claim l further ineluding standoff rings on said end surfaces between said end surfaces and said laminate.

3. A variable resistor as defined by claim l wherein said two cylindrical elements are constructed of electrically nonconductive material.

4. A variable resistor comprising a cylindrical resistance unit, said resistance unit comprising a hollow cylindrical support and a resistance wire wound on said `support with each turn of said wire consisting of foul' successive portions, a first portion located on the outside of said support, a second portion extending around one edge of said support, a third portion located on the inside of said support, and a fourth portion extending around the opposite edge of said support, first and second hollow cylinders, said cylinders having fiat substantially coplanar surfaces at corresponding ends, said cylinders disposed in concentric relation and defining a narrow annular space therebetween, said resistance unit disposed in said annular space with the second portions of said wire turns located proximate to said flat surfaces, means for connecting said resistance wire to a voltage source, a exible resilient contact laminate of annular shape secured to said coplanar end surfaces in bridging spaced relation to said resistance unit, said contact laminate having a conductive surface on one side facing said resistance unit and a nonconductive surface on the other side, means for connecting said conductive surface to an electrical circuit, and means for selectively pressing different portions of said contact laminate toward said resistor unit whereby 8 said conductive surface will contact different turns of said resistance unit.

5. A variable resistor comprising two concentric cylinders dening a narrow annular space therebetween, a substantially cylindrical resistance element located in said annular space and supported by said: cylinders, said resistance element having a resistance wire wound about a at cylindrical card, said resistance element positioned so that one edge of said card is substantially ush with vcorresponding end surfaces of said cylinders, a resilient flat annular contact laminate secured to said corresponding end surfaces in spaced bridging relation with the turns of said wire on said longitudinal edge, said laminate having a conductive layer facing said resistance element, and terminal means for connecting said resistance element and said conductive layer to an external circuit.

6. A variable resistor comprising two concentric cylinders defining a narrow annular space therebetween, a substantially cylindrical resistance element located in said annular space and physically attached to at least one of said cylinders, said resistance element comprising a flat cylindrical card and a resistance wire wound about said card, said cylindrical resistance element concealed by said cylinders with substanially only one edge thereof exposed for engagement by a contact element, said resistance element positioned so that said one edge is substantially Hush with corresponding end surfaces of said cylinders, and a resilient fiat contact laminate secured to and supported by said corresponding end surfaces, said laminate comprising a resilient backing and a resilient conductive contact layer attached to said backing, said flat contact laminate bridging said annular space and disposed in spaced relation with the turns of said wire on said edge.

7. A variable resistor as defined by claim 6 wherein the inner one of said two concentric cylinders is provided with a peripheral flange, and further wherein the outer of said two cylinders is provided with an annular shoulder which is engaged by said flange, said ange and shoulder cooperating to align said end surfaces in a common plane.

References Cited in the file of this patent UNITED STATES PATENTS 1,672,123 Hartranft June 5, 1928 2,862,088 Mairs Nov. 25, 1958 2,884,506 Graustein Apr. 28, 1959 2,896,183 Nebel July 21, 1959 FOREIGN PATENTS 396,877 Great Britain Aug. 17, 1933 UNITED STATES PATENT OFFICE CERTIFICATE OE CORRECTION Patent Noa 3,070,768 December 25 1962 William L Mairs It is herebj)r certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as Corrected below.

Column 7.v line 30, beginning with "4., A variable resistor" strike out all to and including "external eireuitl" in line l, eolumn 8, comprising claims 4 and 5; Column 8, line I7, for read 4 u; line 18, after "therebetween," insert the inner one of said two cylinders having a peripheral flange and the outer one of said two cylinders having an annular shoulder whieh is engaged by said flange, said flange and shoulder cooperating to align Corresponding end surfaces of said cylinders in aA common planeV 3; line 27(JV after "with" insert Said --5 line 35 beginning with "7 A variable resistor" strike out all to and including "Common plane' in line 4l, comprising claim 7; in the heading to the printed specification, line 7xI for "7 Claims." read 4 Claimse Signed and sealed this 20th day of August 1963.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3,070,768 December 25, 1962 William J Mairs It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 7.y line 30, beginning with "4., A variable resistor" strike out all to and including "external circuit," in line 16, column comprising claims 4 and 5; column 8 line 17, for "6 read 4 q line 18, after "therebetween," insert the inner one of said two cylinders having a peripheral flange and the outer one of said two cylinders having an annular shouldern which is engaged by said flange, said flange and shoulder cooperating to.y align corresponding end surfaces of said cylinders in a, common planeq --3 line 27 after "with" insert said --g line 35, beginning with "7:, A variable resistor" strike out all to and including "common plane." in line 4l, comprising claim 7; in the heading to the printed specificationwk line 7v for "7 Claims." read 4 Claims,

Signed and sealed this 20th day of August 1963.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Commissioner of Patents Attesting Officer 

